FRP molded articles are used as bath-tubs, hulls for small-sized ships, bodies for cars, and so forth because of high strength thereof. With these FRP molded articles, however, it is necessary to increase the amount of glass fiber being used for reinforcement at areas that are to be under heavy load, and furthermore, to increase the wall thickness at the areas. This naturally leads to an increase in production costs. Furthermore, since this results in the division of the molded article into a part where the wall thickness is large and a part where the wall thickness is small, which are different in thermal conductivity from each other, internal strains are produced in the molded article by the difference in rate of curing between the thick wall part and thin wall part during cure molding. These internal strains give rise to the problems that the molded article is deformed and unevenness is locally produced.
Therefore, at areas where reinforcement is particularly required, e.g., the bottom of a bath-tub and the bottom of a small-sized ship, a reinforcing material, e.g., a wooden plate and a resin concrete plate, is incorporated into the FRP molded article.
When a wooden plate is used as a reinforcing material, it is necessary to bore a number of holes in the wooden plate since the flow of a resin is hindered at both sides of the wooden plate in a mold. Furthermore, since the wooden plate contains a considerable amount of air in wood fibers, the air leaves the wooden plate at the step that a thermosetting resin is injected and cured, forming air bubbles in the thermosetting resin. In order to prevent the formation of air bubbles, in some cases, the whole surface of the wooden plate is previously coated with an unsaturated polyester resin. It is, however, very difficult to conduct such surface coating perfectly, and it is not possible to completely prevent the formation of air bubbles. Moreover, it suffers from the disadvantage that when load is applied repeatedly, cracks are formed between the wooden plate and FRP material (glass fiber plus molding resin), and ply-separation occurs therebetween.
In addition, the wooden plate has disadvantages in that it is seriously deformed by changes in temperature, humidity, etc., and in that it is, as described above, necessary to bore a number of holes in the wooden plate, and when the wooden plate is not placed at the correct position, it contacts with the surface of a mold, scratching and damaging the mold.
In order to overcome the above described defects of the wooden plate, it has been proposed to use a resin concrete plate in place of the wooden plate. The resin concrete plate is composed of 20% by weight or less of a thermosetting resin, 50 to 80% by weight of a fine aggregate, and 30% by weight or less of a super fine aggregate. It is used mainly in place of cement concrete, and has a specific gravity as high as about 2 (even for light weight resin, the specific gravity is 1 or more) and a modulus of elasticity as high as about 2,000 kg/mm.sup.2, and is water-tight. The use of such a resin concrete plate as a reinforcing material for the FRP molded article undesirably increases the weight of the resulting molded article, deteriorates the fluidity of a thermosetting resin on the upper and lower surfaces of the resin concrete plate as in the case of the wooden plate, requires the formation of a number of holes in the plate as in the case of the wooden plate, and furthermore, produces cracks between the resin concrete plate and FRP material and causes ply-separation therebetween.
Japanese Patent Application (OPI) No. 155617/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses an improved method in which after an FRP molded article is previously formed, a resin concrete plate is laminated thereon at an area to be reinforced, and the same material as for the FRP molded article is applied on the surface of the resin concrete plate to form an overlay layer. Even with the molded article produced in accordance with the method, however, cracks are produced between the FRP material and the resin concrete plate, and ply-separation occurs between the FRP material and resin concrete plate.